ORBIT, in astronomy, is the path described !n space by a heavenlybody in its revolu tion round its primary.* The path so described is of tin elliptic form, and would be accurately an ellipse were it not for the disturbing influence of the other heavenly bodies. )`..;ce PERTURATTONS. The conplete determination of a planet's orbit is of the last importance to astronomers, as it enables them to predict the planet's place in the heavens at any period, and thus determine the exact date of eclipses of the sun and moon, of tran sits and occultations of the planets, and of the appearances and disappearances of comets. For the determination of a planet's orbit, it is necessary to know three things: 1. The situation of the plane of the orbit in space; 2. the position of the orbit in this plane; and 3. the situation at a given epoch, and rate of motion, of the planet in its orbit. Since the plane of Urn ecliptic is for convenience taken as the reference plane, the posi tion of the plane of a planet's orbit is known when its inclination to the plane of the ecliptic (1). and the line of intersection of the two planes (2). mire• known. Since the sun, which is the focus of the planetary orbits, lies in this line of intersection, the orbit cannot lie wholly above or below the plane of the ecliptic, lint must cut it in two points, called kilos (q.v.), and the position of the line of intersection, or line of nodes, is generally given in terms of the longitude (or angular distance) of the ascending node, reckoning from the equinox. The situation of a planet's orbit in its plane is determined
when we know its form (3), size (4), and the position of its major axis or line of aspides (5). The size and form of the orbit depend upon the length of its major and minor axes, but astronomers prefer to employ the major axis and eccentricity (see ELLIPSE); and the position of the major axis is known by determining the heliocentric longitude of its perihelion (i. e., the extremity of it which is nearest the sun). To complete our knowledgeof a planet's motion, all we now require are the epoch of its appearance at some determinate point of its orbit, say at the perihelion (6), and the velocity of its. motion in its orbit (7), for when this last is known, the law of areas, as given in Kepler's second law, enables us to determine the position of the planet in its orbit at any future period. These seven facts, the possession of which gives us a complete clue to a planet's motion, are called the seven "elements of a planet's orbit." What has been here stated concerning the planetary orbits, is equally true of the comets and satellites, though, in the case of the latter, the effect of disturbing forces is so great as to produce a considera ble change of the elements in one revolution.